The officiating rabbi at a wedding, known in Hebrew as the mesader kiddushin, has two primary functions:
The first and primary function of the rabbi is to ensure that the entire wedding is properly conducted according to Jewish law and custom. He makes sure that all the "paperwork," i.e. the ketubah (marriage contract), is in order; that the marriage witnesses meet all halachic criteria; and that all other parts of the ceremony conform to halachic standards.
Secondly, there is a blessing that is technically the groom's responsibility to recite before placing the wedding ring on the bride's finger. Since the groom will likely be too nervous at this juncture to properly pronounce the blessing, the rabbi recites it in his stead.
Please visit our Wedding & Marriage Section for more information on Jewish weddings.
I hope that I've been helpful today. Please feel free to reply.
Yours truly,
Rabbi Menachem Posner

Interesting I had an orthodox wedding in 1999. Very shortly after I the bride asked for a divorce. My husband was extremely controlling and abusive leaving me alone while he went out to bars at night with his friends. I have never gotten a dime from him after I spent hundreds of thousands of dollars on the wedding. The rabbi that married and was a witness on ketubah never agreed to help me.